ND ISOTOPIC EVIDENCE FOR ENHANCED MAFIC WEATHERING LEADING TO ORDOVICIAN COOLING

It remains unclear whether waning of the volcanic degassing CO₂ source or enhancement of the mafic (Ca, Mg-silicate) weathering CO₂ sink, or both, caused global cooling leading to the Ordovician greenhouse–icehouse transition. Here we use a uniquely age-constrained and integrated Middle–Late Ordovician (470–450 Ma) continental weathering (⁸⁷Sr/⁸⁶Sr and ε_Nd(t)) and paleotemperature (δ¹⁸O) isotopic proxy dataset from bulk carbonate and conodonts of the Antelope Range, central Nevada to investigate the role of mafic weathering on Ordovician cooling. ε_Nd(t) measurements of the acid-soluble portion of bulk carbonate samples demonstrate a steady pre-shift baseline at ε_Nd(t) = −18 to −19 followed by a sharp inflection at 463 Ma leading to ε_Nd(t) = −12 ± 4 at ca. 455.5 Ma. These seawater ε_Nd(t) measurements align well with detrital (shale) ε_Nd(t) measurements in Taconic foreland basins, indicating that the shift was caused by a change in continental weathering source lithology. ⁸⁷Sr/⁸⁶Sr of bulk carbonate and conodont apatite align well with conodont ⁸⁷Sr/⁸⁶Sr measurements across Laurentia, decreasing from 0.70881 at 471 Ma to 0.70797 at 453 Ma with a sharp inflection at 463 Ma marking one of the most prominent periods of seawater ⁸⁷Sr/⁸⁶Sr change in the Phanerozoic. Published δ¹⁸O measurements of conodont apatite from the Antelope Range section show a ~2.7‰ increase spanning 465–457 Ma, which corresponds to a ~11 °C cooling and aligns well with globally compiled conodont δ¹⁸O measurements. The coeval shifts in seawater ε_Nd(t) and ⁸⁷Sr/⁸⁶Sr to more juvenile values (higher ε_Nd(t), lower ⁸⁷Sr/⁸⁶Sr) at 463 Ma are roughly coincident with an inflection in global δ¹⁸O to higher (cooler) values captured in the Antelope Range section at 460 ± 1 Ma, suggesting that increased weathering of mafic rocks in the Taconic mountains forced a period of global cooling. We use a published ⁸⁷Sr/⁸⁶Sr and pCO₂ mass balance model to simulate global CO₂ fluxes during the Ordovician. A 25% increase in a global weatherability factor at 463 Ma causes a decrease in pCO₂ from 11.7 PAL to 4.7 PAL (present atmospheric level; 1 PAL = 280 ppmv CO₂) at the end of the Ordovician. Together, our geochemical and modeling results suggest that CO₂ drawdown from mafic weathering was a primary driver of Middle–Late Ordovician cooling leading to end-Ordovician glaciation.